1. Field of the Invention
The present invention relates to a method for separating a release layer. Specifically, the present invention relates to a method for separating a release layer including various devices. And more specifically, the present invention relates to an electric appliance including a display apparatus as typified by a liquid crystal module, an EL (electroluminescent) module, or the like, as one of components.
2. Related Art
In recent years, a technique for forming a TFT composed of a semiconductor thin film (with a thickness of approximately from several to several hundreds nm) formed over an insulating surface of a substrate has attracted attention. TFTs are used for an electric device such as IC, an electro-optic device, or the like, and are especially developed as a switching device or a driver circuit of a display apparatus.
In such a display apparatus, a glass substrate and a quartz substrate are generally used, however, they have disadvantages such as being fragile and heavy. In addition, the glass substrate and the quartz substrate are difficult to grow in size for mass-production. Hence, it is attempted that a TFT device is formed over a substrate having flexibility as typified by a flexible plastic film (a plastic substrate).
However, in case of using a high performance polysilicon film for the active layer of a TFT, a high temperature process at several hundreds degrees is required for a manufacturing process, so that a TFT cannot be directly formed on a plastic film.
Therefore it has already been proposed a separating method for separating a release layer, which is formed over a substrate via a separating layer, from the substrate. For example, a separation layer formed of amorphous silicon (or polysilicon) is formed, and hydrogen contained in the amorphous silicon is released by laser light irradiation through a substrate to produce a space, then the substrate is separated (See reference 1: Unexamined Patent Publication No.10-125929). In addition, Reference 2 (Unexamined Patent Publication No. 10-125930) discloses that a liquid crystal display apparatus is completed by pasting a release layer (referred to as a transferred layer in the reference) to a plastic film by means of the technique disclosed in Reference 1.
However, the above-described method requires a substrate that is very transparent to light, and comparative large amount of laser light irradiation for energizing enough to release hydrogen contained in amorphous silicon, so that a problem becomes arisen that a release layer is damaged. In addition, according to the above described method, in case of forming a device over a separation layer and carrying out heat treatment or the like at high temperature during a manufacturing process, there is a threat that separation is inadequately carried out even if a laser light is emitted to a separation layer since hydrogen contained in the separation layer is decreased due to the heat treatment. Accordingly, processes after forming a separation layer may be restricted in order to keep the amount of hydrogen contained in the separation layer. There is the description in above references that a light-shielding layer or a reflection layer is provided in order to prevent a release layer from being damaged, in such a case, it is difficult to manufacture a transparent liquid crystal display apparatus or a light-emitting display apparatus of bottom emission type.